Grease Pencil why are black strokes gray, and how to make them black for a rendered image?
With your Grease Pencil object selected in Object Mode, go to the Modifier tab in the Properties Panel and click on Add Grease Pencil Modifier at the top.



You will see a few options. One of them is Opacity. Choose it and select your Grease Pencil's current material under the Material dropdown, and then its Layer under the Layer dropdown.
Increase the Factor by more than its current value of 1. 



The result is a completely opaque render (Before and After)



I don't know why this works, nor why it's needed in order to achieve this. But I wasn't able to get an opaque render result any other way.